If you like transistor amplifier. You may also like VU meter using the transistor. They are one type of an Analog VU meter circuit.  Also, It works with an integrated amplifier circuit. To show a level of the sound signal by flashing light of LED display. A rhythm of the music with LED that.

Why should use transistor VU meter

You may have a question. Is it better than IC? I cannot tell you. It is best VU meter. But I can show you the transistor is so great at all time.

• Easy to get them—Although they are quite ancient. But it is still always popular. For example, 2SC458 transistor, now The factory quit producing it. But we can use other transistors instead like 2SC828, 2SC945 etc.
• Cheap—They are inexpensive when compare with IC. Although we need to use a lot of transistors. If it breaks out. You can fix it one by one, do not change all. It is different the IC. You need to change IC only.
• Simple circuit—suitable for learning basic electronics

5 LED VU Meter Circuit using transistors

Look at the circuit. Here is a very simple VU meter. It has only the necessary parts. When entering the power supply to the circuit. Then, connects an input of VU meter to the output of the amplifier.

Next, four diodes D1, D2, D3, and D4 pass input audio signal.  They have 2 functions.  First, rectify the audio signal, AC voltage to DC voltage. Second, divide the audio signal to gradually decrease, respectively.

To forward to each transistors Q1-Q4 through Resistors, R5-R8.  They limited bias current to the base of  Q1-Q4 to increase current up to each collector.

To drive LED1-LED4 to show the level of the signal. The LED1 displays audio level a least light. And the LED4 shows the most level signals.

I think you may not understand I said.  Back…reading.

Each diode has voltage across is 0.6V. Suppose the signal is 1.8V.

It causes the current can flow to three diodes, D1, D2, D3,  So, LED1-LED3 glow up. But LED4 and LED5 will go out.

Note: likewise you can see the second circuit below that too LEDs.

25 LEDs VU meter Circuits (Increase LED)

The weakness of the previous circuit is that it can not display low voltage. So, we need to improve it.  In the circuit below. It has a voltage doubler circuit, C1, C2, R1, R2. They approximately double an incoming AC voltage.  Also, the output is DC voltage.

Make the audio signal into more voltage. So, all transistors can drive all LEDs to emit light much more.

How it works

First of all, when applying the power supply to the circuit. And VR1 passed the input signal and adjust the sound more or less.  Then, flows through the C1 signal coupling.

Next, the signal goes into the circuit to increase voltage with D1 and D2.  Which both diodes rectifier AC to DC with positive and negative together.  Then, DC voltage comes to C2 to filters into smooth DC voltage.

The first part of the signal will flow through R1 to the base of  Q1. It works to drive LED1-LED5 to light up.

The remaining signal is sent through diodes D3, D4 and D5. This will serve to break the sound signal to gradually decrease, respectively.
To forward to the transistors Q2-Q4, each level of the signal strength coming.

Then expand to the LED current to light the rhythm music.

20 LED VU meter schematic

This is 20 LED VU meter schematic is designed for who want to have a circuit that can drive LED up to 20 pcs. (Stereo 40 LEDs) Which the circuit using a generally IC will can not directly connect to uses.

In addition to this project is very easily circuit and also can modify to drives Solid State Relays for easily control a larger lamp. As you see the complete circuit of the projects in Figure 1.

The working of this circuit.

An input signal form power amplifier is passed through to VR1, to adjust the amplitude of this input signal in the proper level.

Then, the signal is sent to the voltage doubler rectifier circuit that consists of C1, C2, D1, D2 to the circuit. All transistor Q1-Q10 that together into the voltage compare circuit.

Figure 1 The 20 LED cheap electronic VU-meter using transistor circuit.

Can see that bass circuits of each transistor will be set a bias with D3-D40 provided to have voltage bias is different for every 2 volts, respectively.

Thus, transistor Q1 will start work at lower signal and Q20 will work on the strongest signal.

The section of the circuit is in the dash as the shortening of the circuit is repeated. Consisting of Q4-Q18.

Build VU meter using transistors

We Assembly the circuit on to PCB lay as Figure 2 correctly. So must be extra careful is. Devices with terminals such as Diode, Electrolytic capacitors, and LED will are connected match terminals only.

Figure 2 the PCB layout and the components layout of the 20 LED cheap electronic VU-meter using transistor project

When the assembly is completed, then to test to provide the voltage power supply to this project. While connects the input signal from a speaker of a power amplifier and next turn on it at highest audio, then adjust VR1 to LED glow fully all LED. Now already have fun.

The components List
Q1-Q20: 2SC828,C945,C1815, 45V 0.1A, NPN Transistor
D1-D40: 1N4148, 75V 0.15A Diodes
R1-R20: 15K, Resistors 0.25W +/- 5%
VR1: 10K, potentiometer POT
C1: 33uF 50V, Electrolytic
C2: 3.3uF 50V, Electrolytic
L1-L20—LED 2 x 5 mm.

Note:
If you want to see more image of VU meter circuits. Please read below.

Great 20 LED VU meter schematic

The complete circuit for VU-meter.

Note: This circuit worked well as Video below: